Process of sulphite cellulose digestion



Patented Dec. 19, 1933 UNITED STATES PROCESS OF SULPHITE CELLULOSE DIGESTION Gustaf Haglund, Stockholm, Sweden, assignor to Patentaktiebolaget Griindal Ramon,

liolm, Sweden Stock- No Drawing. Application April 10, 1931, Serial No. 529,299, and in Sweden January 9, 1931 14 Claims.

In the digestion of cellulose according to the sulphite process the active chemicals of the usual cooking liquor are: acid or neutral sulphites, e. g. of sodium, calcium or other alkali or alkaline earth metals, in conjunction with supplemental sulphur dioxide which may be referred to as free sulphur dioxide as distinguished from that supplied in combination in the acid or neutral sulphite salts.

The digestion is generally carried out with the aid of a cooking liquor previously prepared and containing these chemicals in'fixed proportions, and as a consequence all the constituents have the opportunity of concurrently acting upon the crude cellulose material in the digester from the beginning to the end of the digestion.

For this reason it is not possible to observe the separate reaction of the various chemicals upon,

the crude material. The reactions occur without interruption and the actual course of the reactions caused by the principal reagents can only be a subject of conjecture. The first result of the reactions, however, is that the incrusting substances of the crude material are dissolved and the cellulose fibres freed.

To explain the nature of the reactions involved in the digestion of sulphite cellulose a theory has been advanced and commonly adopted according to which the following two principal reactions are considered to occur.

The primary reaction is considered to be principally characterized by the action of the free sulphur dioxide upon incrusting substances of the crude material whereby rather unstable and difficultly soluble lignin-sulphonic acids and other splitting" products are formed, as the result of what may be termed an acid hydrolysis.

The secondary reaction is considered to be principally characterized by a combination of the base of the sulphurous acid salt provided in the cooking liquor, e. g. alkali metal sulphite, with the organic acids from the acid hydrolysis, in the main lignin-sulphonic acids, whereby stable alkali metal salts of the same are formed.

Besides these two reactions a number of other reactions which tend to affect the digestion adversely may take place. It frequently occurs that a dark colored, incompletely digested cellulose product results. This is probably in part ascribable to the deposition on the cellulose of insoluble reaction products of the lignin-sulphonic acids or other constituents, which products are not taken up by the base of the sulphite salts; and

The present invention relates to a process of sulphite cellulose digestion which will avoid the reactions unfavorable to the digestion, and at the same time promote a more complete disintegration of the crude cellulose material and higher speed of digestion.

As the result of my investigations, I have reached the conclusion that in the usual sulphite cellulose digestion processes, due consideration has not been given to the various reactions upon the crude cellulose material presented by the alkali sulphites on the one hand and the free sulphur dioxide on the other hand, and to the importance of not permitting these reactions to interfere with one another during the digestion period.

Although only incomplete knowledge is available as to how digesting solutions of sulphites or bisulphites or mixtures of these salts, which do not contain anyfree sulphur dioxide, react on the crude cellulose material, it has however been established that the crude material takes up and combines with sulphite or bisulphite in one chemical form or another. After digestion at e. g. 120 C. with such solutions having a fixed content of these salts, the crude material is softened and is at the same time permeated by a certain quantity of the solution, with which it reacts. In the solution remaining, the content of salts is diminished considerably, which proves that part of the salts have been combined with the crude material in some chemical compound. It is found however, that even if the digestion is carried very far a disintegration of the cellulose material of the same character as when free sulphur dioxide is present in addition cannot be effected. It is also found on the other hand that the formation of undesirable, products such as sulphuric acid, sulphur, thiosulphates, etc. has not taken place.

From this latter fact the important conclusion can be drawn that the formation of" these objectionable products, is intimately related to the use of the free sulphur dioxide.

However, it is necessary to add free sulphur plete disintegration of the crude material and secure a pure cellulose possessing the properties that are considered desirable for sulphite cellulose.

In accordance with my invention I have found that the problem of these undesirable effects which accompany the use of the free sulphur dioxide may be effectively solved by carrying out the sulphite cellulose digestion in a two-stage cooking operation, wherein a cookingof the cellulose material with the sulphite and/or bisulphite salts without the free sulphur dioxide constitutes the first stage, and a subsequent cooking of the resulting cellulose material with the 105 dioxide if one is to obtain a substantiallycom free sulphur dioxide constitutes the second stage.

The following description of a specific procedure will illustrate how the digestion of sulphite cellulose may be carried out in practice according to the above described separation of the digestion into one cooking stage for the sulphites and/or bisulphites, alone, and a second, subsequent cooking stage for the free sulphur dioxide, to secure a complete disintegration of the crude material with the production of cellulose and waste liquor.

To a digester filled with wooden chips or any other cellulose-containing material suitable for the digestion, a solution is added, containing for example, alkali metal sulphites, bisulphites or a mixture of these salts, whereupon the digestion is started. The solvent used is preferably ordinary water, although waste liquor, and/or wash water from a previous digestion may be utilized if desired. The solution, which does not contain the free sulphur dioxide, may be preheated before being added to the digester so as to conserve heat and time. The sulphite digestion according to the present method is materially facilitated by using a forced circulation of the liquor, in conjunction with indirect heating thereof externally of the digester to avoid dilution. This may be accomplished by pumping off the liquor in the lower portions of the digester, passing it over steam coils, and returning it at the top of the digester in the form of a spray. Utilizing such procedure, the temperature in the digester is raised to, say 120 C. at an accelerated speed, and this temperature is then maintained for one or more hours, the period varying according to the character of the crude material used and other factors involved in the particular operation.

As dry chips can take up practically their own weight of the liquor, the solution of sulphurous acid salt, for example, sodium bisulphite, used for the preliminary partial digestion of the chips, may be provided with such a high concentration of the sodium bisulphite, that before the addition of the solution of free sulphur dioxide, the chips contain the quantity of bisulphite required for carrying out the digestion process.

For example, the solution of sodium bisulphite added to the digester may be made of such concentration that a quantity of solution corresponding to only one-fourth of the free volume of the digester will contain an amount of bisulphite sufficient for carrying out the treatment. In using such a solution, the quantity introduced into the digester may either correspond approximately to one-fourth of the free volume of the digester, in which case there will be no subsequent removal of the solution, or if desired the digester may be filled more or less completely with such solution. In the latter case, however, after the partial digestion with the concentrated bisulphite solution has been completed, the excess solution should be tapped off from the digester, at least to the extent that sufilcient space will be provided for the succeeding solution of sulphur dioxide, and to a further extent if desired.

As a result of the treatment of the cellulose material in the manner described, the stock has absorbed and become permeated by the sulphite solution, has become softened, and the incrusting substances of the crude material have entered into chemical combination with the sulphites, with a consequent influence upon the subsequent diion with the free sulphur dioxide. While the chemical composition of the incrusting substances has been altered, a disintegration of the crude material of the same character as is the case in the finished product from the usual sulphite digestion wherein free sulphur dioxide is jointly present, has not taken place.

After a suitable partial digestion of the stock as aforenoted, the solution containing free sulphur dioxide, hereinafter referred to simply as the sulphur dioxide solution, is then introduced into the digester. This solution may either be saturated with sulphur dioxide or contain the same in any suitable lesser concentration, depending upon the reactivity desired, the free volume available in the digester and other requirements of the particular operation. The solution may consist simply of an ordinary water solution of sulphur dioxide, or it may consist of a solution of the latter in waste liquor and/or wash water from previous digestions. In practice, however, the use of waste liquor and/or wash water as the carrier for the sulphur dioxide, is much preferable because of its considerable commercial importance in the regenerative treatment of the alkali employed, particularly where alkali metal salts are used. By so doing, the concentration of the by-products in dilute waste liquor or wash water can, by suitable repetition of the operation, be sufliciently increased to permit much more economical recovery. In the recovery of the alkali by incineration, for example, the liquor may be rendered so highly concentrated in organic matter that only a relatively slight and inexpensive evaporation is required and the organic matter will provide more than suflicient fuel for the incineration to recover the alkali, and the heating value of the excess may be diverted to other uses.

When being added to the digester, the temperature of the sulphur dioxide solution should 115 preferably be as low as is compatible with heat economy requirements and the like, in order to minimize as much as possible the liberation of sulphur dioxide gas from the solution as a result of the temperature increase. It is to be noted 120 that ordinarily the digester and its contents will still have a temperature appreciably above 100 C. at the time of addition of the sulphur dioxide solution, and as a consequence, the temperature of the solution will generally be elevated above 125 the point at which all of the sulphur dioxide will be retained in solution. Upon the liberation of sulphur dioxide gas, the pressure in the digester will be increased, and if loss of sulphur dioxide gas during introduction into the digester is to 180 be avoided, the solution of sulphur dioxide will have to be introduced under pressure, by means of a pump, for example.

After the digester has been filled with the sulphur dioxide solution, the forced circulation system and the external heating devices are started again and the temperature in the digester is gradually elevated to and maintained at the point required for carrying the digestion to completion. In the treatment of an easy bleaching pine pulp, for example, an ultimate temperature in the neighborhood of 128 C. gave satisfactory results, although the required temperature and attendant conditions, will vary depending upon the nature of the cellulose material being treated, the quality desired, etc., as will be apparent to one skilled in the art.

During this second digestion step, the other reaction, viz., the acid hydrolysis, is effected by means of the sulphur dioxide with the resulting 15 phase of the digestion requires but a compara-' tively short period of time for bringing about complete disintegration of the cellulose material and obtaining an easy bleaching sulphite cellulose. This second reaction is accelerated to such an extent that the sulphur dioxide solution requires only from five to ten hours for the production of an easily bleached cellulose, half or less than half the time required for the usual digestion treatment.

Moreover, while the exact length of time for the entire operation, that is, for both the preliminary and the secondary treatments, will vary, depending upon. the temperature employed, the extent to which the preliminary partial digestion is carried out, the concentration of sulphur dioxide in the sulphur dioxide solution, the nature of the crude material which is being digested, etc., the total time of digestion is considerably shorter in the present process than in the prior methods.

In reference to the preliminary digestion with the sulphurous acid salts as described hereinbefore, it is to be noted that such digestion may be conducted to cause the effects produced thereby to take place to either a very substantial degree, to only a slight degree, or to intermediate degrees before the sulphur dioxide solution is added, all as the particular circumstances may render necessary or desirable. For example, in certain operations it may be suitable to conduct the preliminary digestion treatment at temperatures somewhat below 120 C. and/or for a short or long period of time; and the same is true as regards the use of temperatures slightly or materially higher than 120 C. with subjection of the crude material to a shorter preliminary treatment, or to a protracted preliminary treatment to secure as extensive a digestion with the sul- K phurous acid salt as is possible.

With respect to the degree and duration of action of the sulphur dioxide solution in the subsequent treatment much the .same situation exists, although the nature of this treatment is dictated to a substantial extent by the nature of the preliminary digestion which has previously taken place.

The degree and duration of action in the preliminary digestion and in the subsequent sulphur dioxide treatments respectively, will depend mainly upon the nature of the crude material being treated, and upon the particular combination of preliminary and subsequent treatments with respect to those factors, which is found most favorable or desirable for the digestion operation involved. In any case, however, the period of contact and manner of conducting the preliminary treatment should be such that the solution of sulphurous acid salt will be given an opportunity to permeate the crude material to a suitable degree before the sulphur'dioxide solution is added, whether or not a substantial.

softening and partial digestion of the crude material by the action of the sulphurous acid salts is also sought.

Apart from the matter of effecting an actual digestion of the crude material to various degrees as in the preliminary treatment hereinbefore described, the process may also be carried out successfully by having the preliminary treatment consist in simply treating the crude material with the solution of sulphurous acid salt at a temperature below that at which appreciable digestion would take place. Thereafter the sulphur dioxide solution is added, the temperature is elevated into the active digestion range, and the digestion is carried to completion.

In carrying out this modified procedure, the solution of sulphurous acid salt, at room temperature or somewhat thereabove, is introduced into the digester charged with the cellulose-containing material to be treated, for example, wood chips, and the chips are exposed to the solution until they have become suitably permeated thereby. This may be accomplished by simply permitting the chips to soak in thesolution for a suitable length of time, or, if desired, by utilizing forced circulation of the solution as in the procedure first described, although in this case the external heating is dispensed with.

After the desired degree of permeation of the chips by the solution has been obtained, the excess solution, if any, may be partially or wholly drawn off as the space requirements of the sulphur dioxide solution or other circumstances may dictate. The sulphur dioxide solution is then introduced into the digester, preferably with the solution at a temperature low enough to assure substantial retention of the sulphur dioxide in solution during the introduction. Thereafter the temperature in the digester is gradually increased to the required degree, either by the use of forced circulation and external indirect heating of the liquor or in any other suitable manner, and the digestion is carried to completion.

Except that the preliminary treatment of the crude material with the solution of the sulphurous acid salt is conducted at a lower temperature, and is not specifically concerned with a distinct preliminary digestion of the material, the entire procedure may be otherwise conducted in substantially the same manner as in the procedure first described. The same salts of sulphurous acid may be used; likewise, the same solvents for the sulphurous acid salt and for the sulphur dioxide viz., ordinary water, waste liquor, etc.; the sulphurous acid salt may be provided in such high concentration in the solution that before the addition of the sulphur dioxide solution, the chips contain the quantity of salt required for the subsequent digestion process, etc.

After the digestion of the cellulose-containing material by the two general procedures noted has been completed or carried to the degree desired, the cellulose is separated from the accompanying liquor, washed, and otherwise treated in the usual manner.

By the process herein described the objectionable results proceeding from unfavorable reactions which take place in sulphite cellulose digestion as heretofore carried out are avoided, the digestion period is materially shortened, a particularly complete disintegration of the crude cellulose material is obtained, waste liquor may be utilized as a solvent for the sulphur dioxide utilized in the digestion with resultant ecconomic advantage and a high grade and practically pure cellulose is produced.

In addition to these advantages, I have found that the process is eminently adapted for the sulphite cellulose digestion of pine, fir, and similar woods which contain material amounts of resinous matter, and results in the production of a very high grade sulphite cellulose therefrom. Heretofore it has not been possible to digest these woods satisfactorily by the sulphite process, and resort to the sulphate process has been necessary for their digestion.

The terms sulphur dioxide solution and free sulphur dioxide as used herein, in reference to the second step of the digestion are intended to include both uncombined sulphur dioxide in true solution in the solvent and sulphur dioxide in the form of sulphurous. acid resulting from chemical combination with water in the solvent.

I claim as my invention:

1. The sulphite cellulose digestion process which comprises, first cooking the cellulose-containing material with a solution of a sulphurous acid salt, then, after withdrawal (if necessary) of a suitable amount of the original solution from the cooking vessel, adding a solution of sulphur dioxide in a solvent of the group: waste liquor, wash water from a similar cooking operation, to obtain a higher concentration of by-products in the waste liquor of the instant operation, and cooking the cellulose-containing material with said solutions.

2. The sulphite cellulose digestion process which comprises, first cooking the cellulose-containing material with a solution of a sulphurous acid salt, then, after withdrawal (if necessary) of a suitable amount of the original solution from the cooking vessel, adding a solution of sulphur dioxide in a solvent of the group: waste liquor, wash water from a similar cooking operation, to obtain a higher concentration of by-products in the waste liquor of the instant operation, cooking the cellulose-containing material with said solutions, separating the cooked cellulose from the waste liquor, and utilizing said waste liquor as a solvent for sulphur dioxide used in a similar subsequent cooking operation.

3. The sulphite cellulose digestion process which comprises, contacting the cellulose-containing material with a solution of a sulphurous acid salt in a digester, forcibly circulating said solution through said digester while maintaining a temperature suitable for the digestion of said material, then, after withdrawal (if necessary) of a suitable amount of the original solution from the cooking vessel, adding a solution of sulphur dioxide, and forcibly cimulating the liquor then contained in the digester through said digester while maintaining a temperature suitable for the digestion of the cellulose-containing material.

4. The sulphite cellulose digestion process which comprises, first cooking the cellulose-containing material with an acid solution of a sulphurous acid salt at a suitable elevated temperature in a digester, then conducting the introduction of a sulphur dioxide solution into said digester underpressure to avoid loss of sulphur dioxide liberated, then elevating the temperature in the digester, and cooking the cellulosecontaining material with said solutions.

5. The sulphite cellulose digestion process which comprises, first cooking the cellulose-containing material'with an acid solution of a sulphurous acid salt, the total sulphur dioxide content of such solution not to exceed approximately the amount which would be required theoretically for bisulphite of the metal or metals present, and then cooking the cellulose-containing material with a solution of sulphur dioxide.

6. The sulphite cellulose digestion process which comprises, first cooking the cellulose-containing material with an acid solution containing water-soluble acid salt of sulphurous acid, comprised in the group: alkali metal bisulphite, alkaline earth metal bisulphite, mixtures of bisulphite of said metals, and mixtures of bisulphite of said metals with normal sulphite of said metals; the total sulphur dioxide content of said solution however comprised not to exceed approximately the amount which would be required theoretically for bisulphite of the metal or metals present, and then cooking the cellulose-containing material with a solution of sulphur dioxide.

7. The sulphite cellulose digestion process which comprises, first cooking the cellulose-containing material with a solution of sodium bisulphite, the total sulphur dioxide content of such solution not to exceed approximately the amount required theoretically for the bisulphite, and then cooking the cellulose-containing material with a solution of sulphur dioxide.

8. The cellulose digestion process which comprises cooking cellulose-containing material with a bisulphite solution containing no free sulphur dioxide, and subsequently cooking the material with a solution containing free sulphur dioxide.

9. The cellulose digestion process which comprises cooking cellulose-containing material with a solution containing monosulphite and bisulphite and containing no free sulphur dioxide, and subsequently cooking the material with a solution containing free sulphur dioxide.

10. The cellulose digestion process which comprises cooking cellulose-containing material with a solution of a salt of sulphurous acid containing no free sulphur dioxide, and subsequently cooking the material at a higher temperature with a solution containing free sulphur dioxide.

11. The cellulose digestion process which comprises cooking cellulose-containing material with a bisulphite solution containing no free sulphur dioxide, and subsequently cooking the material at a higher temperature with a solution containing free sulphur dioxide.

12. The cellulose digestion process which comprises cooking cellulose-containing material with a solution containing monosulphite and bisulphite and containing no free sulphur dioxide, and subsequently cooking the material at a higher temperature with a solution containing free sulphur dioxide.

13. The cellulose digestion process which comprises cooking cellulose-containing material with a bisulphite solution containing no free sulphur dioxide, and subsequently cooking the material with a solution containing free sulphur dioxide and at least a portion of the original cooking solution.

14. The cellulose-digestion process which comprises cooking cellulose-containing material with a solution containing monosulphite and bisulphite and containing no free sulphur dioxide, and subsequently cooking the material with a solution containing free sulphur dioxide and at least,

a portion of the original cooking solution.

GUS'I'AF HAGLUND. 

